Mcleod mcadie stephen



1956 A. MOLEOD MCADIE STEPHEN 2,732,942

SCREENING Filed Oct. 30, 1951 2 Sheets-Sheet l F/GZ.

lnvenl or ALEXANDER MCLEOD MCADE STEPHEN M w, MF W A tlorneys Jan. 31,1956 Filed Oct. 30, 1951 A. MOLEOD MOADIE STEPHEN v 2,732,942

SCREENING 2 Sheets-Sheet 2 ,F/GB.

I nventor:

A lexander McLeod McAdie Sfep/aen United States Patent SCREENINGAlexander McLeod McAdie Stephen, Norton-on-Tees, 5

England, assignor to Imperial Chemical Industries Limited, a corporationof Great Britain Application October 39, 1951, Serial No. 253,76?

Claims priority, application Great Britain November 17, 1959 10 Claims.or. 209-323 This invention relates to mechanically operated screens. Itis applicable, e. g., to rotating or to shaking screens, which last mayuse reciprocating and gyratory motion.

According to the present invention there is provided a method ofmaintaining a mechanical screen in more efficient condition Whilescreening material, which comprises providing at least one hollow bodyof such configuration that it is resilient and capable of bouncing, andarranging that the said body or bodies are caused to bounce against ascreening surface or surfaces ofthe screen through the instrumentalityof the motion of the screen so that passage of the material through theapertures of the screen is facilitated. Preferably the hollow bodies arecaused to bounce against the underside of the screening medium. In itspreferred form the hollow body takes the form of a detached unrestrainedcoil spring.

Suitable springs are coil springs shaped in major part so that theimaginary surface in which the outermost diameters of the coils of thespring lie is substantially a surface of revolution having doublecurvature and which when viewed from the exterior is convex. The surfaceis preferably spherical, but may also take other forms, such asspheroidal or ellipsoidal or even paraboloidal. In general the morenearly the surface approaches spherical the better the results.

The invention is of particular value in its application to mechanicallyoperated screens having one, or having two or more decks. In screens ofthis type there are commonly provided superimposed units, each fittedwith a cloth having a mesh of lesser aperture than the cloth above it,and the whole is shaken by a suitable mechanism. The movement may begyratory or oscillating and is commonly effected by means of aneccentric drive. For example, the eccentricity employed may be PA"giving a total longitudinal throw of 3 /2" and there may be 200 cyclesper minute. The invention is of special value where the amplitude of themovement is appreciable, for example /2" and more.

In order to keep the meshes of such screens free,

especially when the material being screened is fine, it has 5 been thepractice to make each deck in the form of a unit comprising a screeningsheet of desired mesh and an auxiliary sheet of coarser mesh situatedbeneath the first and spaced therefrom; to provide between eachscreening sheet and its auxiliary sheet a frame divided into a number ofcompartments, which rests on the auxiliary sheet; and to place in eachcompartment a numberof solid rubber balls. While the compartments may beof any suitable shape, for example diamond, they are preferably oblongor square and formed by longitudinal and 60 transverse members. Forcompartments which are 15" square there may be 3 or 4 balls in eachcompartment, for example.

Preferably each frame extends close up to each of the cloths betweenwhich it lies, and the transverse members of the frame are shaped sothat they each present interiorly to the compartment a plane faceinclined at an obtuse angle to the lower, i. e., the auxiliary cloth.This angle may be for example between 120 and Under the effect of themotion of the screen the balls are projected, come into contact with theinclined faces of the transverse members and rebound therefrom againstthe underside of the cloths, thereby shaking the latter and thusfacilitating passage of the material through the cloths and tending tokeep them clean.-

However, it has been found in practice with abrasive and/or hotmaterials that the balls wear and lose their efficiency. This results inincreased costs due to replacement of the balls, and dismantling of thescreen. Although we have tried solid and hollow balls made from a largenumber of materials, we have been unable to find any that gavesatisfactory performance under these conditions.

It has also been previous practice to make the aforesaid framework ofwood on account of the lightness it confers, but screens with suchframeworks sulfer from the further disadvantage that the wood soon failswhen they are used for the screening of hot and abrasive materials. Thescreening of hot and abrasive materials is an operation of widecommercial use, since it is common practice, on account of the economiesachieved, to screen dried materials immediately after drying, and thematerial may be at a temperature or", for example, up to C. or more.This applies, for example, to the drying of fertilizers or ores.

According to a preferred feature of the invention, therefore, there isprovided a method of maintaining a one, two, or multi-deck vibratingscreen in more efficient condition whilst employed in screening materialwhich comprises providing each deck with a number of compartments on theunderside by means of suitably disposed metal struc tural members andproviding in at least some of the compartments at least onedetachedrnetal coil spring shaped in major part so that the imaginarysurface in which the outermost diameters of the coils of the spring lieis substantially a surface of revolution having double curvature andwhich, when viewed from the exterior is convex, and causing the saidcoil springs to bounce against the underside of the screening clothsthrough the instrumentality of the motion of the screen, therebyfacilitating passage of the material through the apertures of thescreening cloths. The transverse sectional members, and preferably alsothe longitudinal ones, are shaped so that they present inclined facesinteriorly to each compartment, as already described, from which themoving coil springs can rebound. It will be obvious that with such ascreen having, for example, a total longitudinal throw of 3 /2 andmoving with a frequency of 200 cycles per minute the springs willimpinge against the screens very many times per minute. We have in factfound that their action is very effective in keeping the cloths cleanand in increasing screening rates. It is preferred to have only onespring in each compartment of the framework. Better cleaning of thescreening surfaces may be achieved by the use of several springs in eachcompartment, so long as the springs remain detached from one another.However, on account of the tendency for the springs to become attachedto one another, it is preferred to use one spring in each compartment.The springs also reduce build-up of fine material on the sides of thecompartments.

A suitable apparatus in which the method of the present invention may beconducted is illustrated in Figures 3 and 4 in the accompanying drawing,in which Figure 3 is a longitudinal cross section, illustrating theinvention in a preferred form in its application to a three-deckgyratory screen, and Figure 4 is a perspective view showlight metalframes.

ing one of the compartments in a deck of such a screen from which thescreening sheet has been removed.

Referring to Figures 3 and 4, 1 represents the screening box; 2 a drivehead, driven through shaft 3 by a motor, for'applying the gyratorymotion to the screen; 4 the inlet sleeve; 5, 6 and 7 the three decks;and 8, 9, 10 and 10a the outlet sleeves for the screened material. Eachdeck comprises a screening sheet 11, carried in a frame 12; an auxiliarysheet 13 situated at an appropriate distance, e. g., about 1 /2 to 2 /2"below the screening sheet 11 and carried in the same frame 12; acompartmented metal frame resting on the auxiliary sheet 13 andconsisting of longitudinal and cross members, 14 and 15, respectively;and in each of its compartments a detached spherical spring 16. Thecross members 15 are of trapezium-like cross section, thus presentinginteriorly to the compartment a plane face 17 inclined at an obtuseangle toth'e auxiliary sheet 13. The drive head is of known type andcomprises, for example, a cam driven through bevel gearing, whichimparts a gyratory motion to the driven end of the screen by means of apivot 2a engaging a socket 2b on the screen box. The non-driven end ofthe screen moves in longitudinal slides.

The arrows show the direction of movement of the material beingscreened.

As a further feature of the invention, especially where materials arebeing screened hot, the frame encasing the screens and the membersforming the compartments are made of metal, especially stainless steel.In order to keep down weight and thereby save power, the sectionsforming the compartments should be as light as Possible, e. g., of 20British Standard wire gauge. With such frames it was,'however, foundthat solid balls made of temperature and abrasion resistant material,which were otherwise satisfactory, failed because they dented theDenting results in the balls not bouncing properly on to the cloths. Thecoil springs herein described did not do this, and have maintained theirefiiciency over long periods without apparent deterioration.

, When the material being screened is abrasive it is preferable that thescreening media, springs, casing and frames be of metal, for example, ofstainless steel.

When the material being screened is hot it is preferable that thescreening media, springs, casing and frames be of metal, e. g.,stainless steel.

When the material being screened is a foodstuff it is preferable thatthe screening media, springs, casing and frames beof stainless steel.

When the material being screened is corrosive it is preferable thatthescreening media, springs, casing and frames be of stainless steel.

When a heavy material such as ore is being screened it is desirable thatthe screen, springs and frame members should be robust. This in generalmeans heavier equipment and more powerful driving means.

7 The coils of the spring may be substantially contigubus, butpreferably they are spaced. The configuration,

dimensions and spacing of the coils are chosen so that the materialbeing screened does not build up or cake and so produce a more or lesssolid unit, which would deprive the springs of their advantages ofresiliency and lightness. One way of achieving this is to provideadequate free space between the coils to allow the material to passthrough and not pack the coils. The characteristics of the coils arevaried depending on the mesh size of the screens and on the properties,especially grading, of the product. For general purposes the springs maybe formed, for example, of mild steel, high carbon steel, alloy steels,Monel metal, or bronze, but non-corrosive metals are generallypreferred.

Suitable spherical springs are made of steel rod or wire, e. g., of from0.048 to di diameter. Typical suitable spherical coil springs have thecharacteristics given a similar rubber ball.

in the following table. They weigh about one third of Diameter of wire,Pitch of Weight; Outside diameter of coil, ins. British coils, of 0011,

Standard mms. gms. wire gauge The spherical springs are made by feedingcold rod or wire through a die to a substantially spherical mandril,provided with spiral grooves, under tension and, if ordinary steel isused, thereafter hardening the steel by passing the coil into a cyanidebath at about 150 C. High carbon steels are preferred to mild steels,and austenitic steels, especially stainless steels, are more preferredonaccount of their good corrosion resistance. It is preferred to havethe spherical springs as near true spheres as possible so that they donot assume a dead position, i. e., with the axis of the spring parallelor perpendicular to the screen, but bounce as much as possible. It is anadvantage to bend each end of the coil into a small arch at theextremities of the sphere and to tuck the loose ends inside the spring.

A spring found to be very suitable for screening hot fertilizer at 115C. between the-meshes of a gyratory 3 deck screen having apertures of10, 5 and 0.5 mm. respectively with 12 mm. aperture auxiliary sheetswhich had a throw of 1 /4 at a frequency of 200 cycles, had thefollowing characteristics.

The axial length of the spring was 1%" and the equatorial diameter 1%",and it was formed of A5" stainless steel rod with a pitch of at thecentre, reducing towards the poles.

The invention is illustrated by the following example.

Example approximately lb. Each frame was divided into 40 compartments,in which the transverse faces of the frames were inclined at about tothe lower cloth. The

screen was used toscreen mixed artificial fertilizer which had beendriedand had to be screened at a temperature of 115 C. Originally 3 solidresilient rubber balls were installed in each compartment, making 360balls in all, and the screen was put into service for 6 weeks. Duringthis period the balls became reduced from 2" to /2" diameter andconsequently became ineffective in cleaning the screens. Thisnecessitated replacing 60 balls each week and the stainless steel clothfa'steningshad therefore to be lifted regularly, which necessitated thecomplete dismantling of the screen at least once per week. This meantconsiderable loss of running time and involved considerable expenditure.

Similar results were obtained using solid balls of butylneoprene. Whenmore durable materials, e. g., solid balls of porcelain, hardenedpolished steel, and the mawith scribed above, it was found that thescreens could be run for very much longer time without dismantling, e.g., 3 months, and that their general screening efiiciency was very muchimproved over anything previously obtained. So far as could be observedthere was no deterioration in the springs and practically no loss intheir weight. Moreover, practically no build-up on the inclined faces ofthe compartments took place. Consequently substantially true surfaceswere maintained for rebound of the balls. This was a distinctimprovement over the use of the rubber balls which often compacted themoist powdered fertilizer and caused it to adhere to the inclinedsurfaces of the compartments.

A suitable form of spherical spring according to the invention is shownin elevation in Figure 1 and in plan in Figure 2 of the accompanyingdrawing.

The primary requirements in the hollow body are that it be abrasionresistant, resilient and of low weight.

Instead of a coil spring there may be used other resilient bodies ofsimilar general configuration, e. g., the

bodies formed from spherical, spheroidal, ellipsoidal, or evenparaboloidal, shells of metal by removing a portion or portions of theshell and leaving a resilient residue, e. g., a continuous spiral stripof metal, or a spherical latticework. Alternatively, resilient bodies ofsimilar configuration may be made by joining suitably shaped metallicelements, e. g., by Welding or soldering, or by stamping. While residualelements of substantially spiral strip form are preferred, otherconfigurations may be used provided they are sufficiently resilient toenable them to bounce under the conditions of use in the screens. Thereis, however, a tendency for build-up of fine material to occur onelements formed in these ways. If paraboloids are used, they should bedouble paraboloids.

The hollow bodies are preferably of metal, but where the temperature atwhich screening is conducted and the nature of the material being sievedpermit they may if desired be of such materials as synthetic resins,etc.

I claim:

1. In vibrating and gyratory screens having at least one deck comprisinga screening sheet, carried in a frame, and an auxiliary sheet of coarsermesh, also carried in a frame, situated beneath the first sheet andspaced therefrom, the combination of an open frame, formed with a numberof compartments by longitudinal and cross members, resting on theauxiliary sheet and extending close up to the screening sheet, andsituated in at least some of the compartments at least one detachedmetal coil spring shaped in major part so that the imaginary surface inwhich the outermost extremities of the coils lie is substantially asurface of revolution having double curvature which when viewed from theexterior is convex and therefore adapted for bouncing.

2. In vibrating and gyratory screens having several decks eachcomprising a screening sheet carried in a frame and an auxiliary sheetof coarser mesh also carried in a frame situated beneath the first sheetand spaced therefrom, the combination in conjunction with each deck ofan open frame formed with a number of compartments by longitudinal andcross members, resting on the auxiliary sheet and extending close up tothe screening sheet and situated in all of the compartments at least onedetached metal coil spring shaped in major part so that the imaginarysurface in which the outermost extremities of the coil lie issubstantially a surface of revolution having double curvature which whenviewed from the exterior is convex and therefore adapted for bouncing.

3. The combination as claimed in claim 2 in which the detached metalcoil spring is of high carbon steel.

4. In vibrating and gyratory screens, oscillated at at least 200 cyclesper minute, having at least one deck comprising a screening sheet,carried in a frame, and an auxiliary sheet of coarser mesh, also carriedin a frame, situated beneath the first sheet and spaced therefrom,

the combination of: an open metal frame formed with a number ofcompartments by longitudinal and cross members of light gauge metalresting on the auxiliary sheet and extending close up to the screeningcloth, and situated in each of the compartments at least one detachedmetal coil spring shaped in major part so that the imaginary surface inwhich the outermost diameters of the coils lie is substantially asurface of revolution having double curvature, which when viewed fromthe exterior is convex and therefore adapted for bouncing.

5. The combination as claimed in claim 4 in which the cross members ofeach open metal frame are of a section which presents interiorly to eachcompartment a plane face inclined at an angle of to to the auxiliarysheet.

6. The combination as claimed in claim 4 in which the longitudinal andcross members of each open metal frame are each of a section whichpresents interiorly to each compartment a plane face inclined at anangle of 105 to 120 to the auxiliary sheet.

7. The combination as claimed in claim 4 in which the detached metalcoil spring is of substantially spherical shape.

8. In vibrating and gyratory screens, oscillated at at least 200 cyclesper minute, having at least one deck comprising a screening sheetcarried in a frame, and an auxiliary sheet of coarser mesh also carriedin a frame and situated beneath the first sheet and spaced therefrom,the combination of features in each deck which comprises: stainlesssteel frames and sheets; an open frame formed into a number ofcompartments by longitudinal and cross members of light gauge stainlesssteel resting on the auxiliary sheet, and extending close up to thescreening sheet, and situated in each of the compartments at least onedetached coil spring of stainless steel shaped in major part so that theimaginary surface in which the outermost diameters of the coils lie issubstantially a surface of revolution having double curvature, whichwhen viewed from the exterior is convex and therefore adapted forbouncing.

9. In gyratory screens oscillated at at least 200 cycles per minutehaving at least one deck comprising a screening sheet carried in aframe, and an auxiliary sheet of coarser mesh also carried in the frameand situated beneath the first sheet and spaced therefrom, thecombination in conjunction with each deck of: an open metal frame formedinto a number of compartments by longitudinal and cross members of lightgauge metal, each having sections which present interiorly to eachcompartment a plane face inclined at an angle of 105 to 120 to theauxiliary sheet, resting on the auxiliary sheet and extending close upto the screening sheet; and, situated in each of the compartments atleast one detached substantially spherical metal coil spring adapted forbouncing of which the diameter of the coils of the wire forming the.spring is from 0.048" to A 10. In gyratory screens for use in thescreening of hot materials having at least one deck comprising ascreening sheet carried in a frame, and an auxiliary sheet of coarsermesh also carried in a frame and situated beneath the first sheet andspaced therefrom, the combination of features: screening and auxiliarysheets of metal; metal frames; in conjunction with each deck an openmetal frame formed into a number of compartments by longitudinal andcross members of light gauge metal, each having sections which presentinteriorly to each compartment a plane face inclined at an angle of 105to 120 to the auxiliary sheet, resting on the auxiliary sheet andextending close up to the screening sheet; and situated in each of thecompartments at least one detached substantially spherical metal coilspring adapted for bouncing; the screen being oscillated at at least 200cycles per minute.

(References on following page) References Cited in the file of thispatent UNITED STATES PATENTS Bradford Feb. 27, 1872 Poor May 11', 1886Johnston et al. Jan. 18, 1910 Luxmore Sept. 7, 1915 Chrabkon Oct. 6,1925 Carlson Feb. 25, 1930 Keefer Sept. 5, 1933 Roth Nov. 28, 1933Simpson Mar. 7, 1939 Simpson et a1 Dec. 24, 1940 Schifliin et a1. Jan.6, 1942 FOREIGN PATENTS Switzerland Aug. 24, 1910 France Apr. 28, 1947

